birds) or to the prairie falcon, this temperate steppe population must have preyed on landbirds and mammals rather than the sea and landbirds which make up much of the American gyrfalcon's diet today. Ecology The gyrfalcon was originally thought to be a bird of tundra and mountains only; however, in June 2011, it was revealed to spend considerable periods during the winter on sea ice far from land. It feeds only on birds and mammals, the latter of which it takes more regularly than many other Falco species. Like other hierofalcons, it usually hunts in a horizontal pursuit, rather than with the peregrine's speedy stoop from a height. Most prey is killed on the ground, whether they are captured there, or if the victim is a flying bird, forced to the ground. The diet is to some extent opportunistic, but a majority breed and hunt coinciding with ptarmigan and seabird colonies. Avian prey can range in size from redpolls to geese and can include gulls, corvids, smaller passerines, ducks, sparrows, buntings, doves, grouse, waders, and other raptors (up to the size of Buteos). Mammalian prey can range in size from shrews to marmots (sometimes thrice the weight of the assaulting falcon), and often includes lemmings, mice, voles, ground squirrels, hares, rabbits, and rarely also bats. Gyrfalcons also hunt fish. They are rarely observed eating carrion. Threat from climate change In the early 2000s, it was observed that as climate change began to temper the Arctic summers, peregrine falcons were expanding their range north to parts of Greenland, and competing with gyrfalcons. Although it is specially adapted for high-Arctic life, and larger than the peregrine, the gyrfalcon is less aggressive and more conflict-averse, and so is unable to compete with peregrines, which regularly attack and overwhelm the gyrs. Breeding The gyrfalcon almost invariably nests on cliff faces. Breeding pairs do not build their own nests, and often use a bare cliff ledge or the abandoned nest of other birds, particularly golden eagles and common ravens. The clutch can range from 1 to 5 eggs, but is usually 2 to 4. The average size of an egg is ; the average weight is . The incubation period averages 35 days, with the chicks hatching at a weight of around . The nestlings are brooded usually for 10 to 15 days and leave the nest at 7 to 8 weeks. At 3 to